https://iopscience.iop.org/article/10.1088/1361-6668/ab1c00/pdf

Fe-based superconducting thin films—preparation and tuning of superconducting properties

The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H3S exhibits superconducting transition at Tc = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of Tc>240 K. In this paper, we analyse available upper critical field, Bc2(T), data for LaH10 (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of Tc to the Fermi temperature, Tf, 0.009 < Tc/Tf < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental Bc2(T) data for superconductors in the palladium-hydrogen (PdHx) and thorium-hydrogen-deiterium (ThH-ThD) systems and surprisingly find that superconductors in both these systems also fall into unconventional superconductors band. Taking in account that H3S has the ratio of 0.012 < Tc/Tf < 0.039 (Talantsev 2019 Mod. Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e., Th4H15-Th4D15, PdHx, H3S and LaH10 (in this list we do not include NbTiHx, PtHx, SiH4, and PH3 for which experimental data beyond Tc are unknown), lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors.

Classifying hydrogen-rich superconductors

Iron-based superconductors (FBS) comprise several families of compounds having the same atomic building blocks for superconductivity, but large discrepancies among their physical properties. A longstanding goal in the field has been to decipher the key underlying factors controlling TC and the various doping mechanisms. In FBS materials this is complicated immensely by the different crystal and magnetic structures exhibited by the different families. In this paper, using aberration-corrected scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we observe a universal behavior in the hole concentration and magnetic moment across different families. All the parent materials have the same total number of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies due to different orbital occupancy. Although the common understanding has been that both long-range and local magnetic moments decrease with doping, we find that, near the onset of superconductivity, the local magnetic moment increases and shows a dome-like maximum near optimal doping, where no ordered magnetic moment is present. In addition, we address a longstanding debate concerning how Co substitutions induces superconductivity in the 122 arsenide family, showing that the 3d band filling increases a function of doping. These new microscopic insights into the properties of FBS demonstrate the importance of spin fluctuations for the superconducting state, reveal changes in orbital occupancy among different families of FBS, and confirm charge doping as one of the mechanisms responsible for superconductivity in 122 arsenides.

Orbital occupancy and charge doping in iron-based superconductors

Recently, Snider et al (2020 Nature 586 373) reported on the observation of superconductivity in highly compressed carbonaceous sulfur hydride, H x (S,C) y . The highest critical temperature in H x (S,C) y exceeds the previous record of T c = 280 K by 5 K, as reported by Somayazulu et al (2019 Phys. Rev. Lett. 122 027001) for highly compressed LaH10. In this paper, we analyze experimental temperature-dependent magnetoresistance data, R(T,B), reported by Snider et al. The analysis shows that H x (S,C) y compound exhibited T c = 190 K (P = 210 GPa), has the electron–phonon coupling constant λ e−ph = 2.0 and the ratio of critical temperature, T c, to the Fermi temperature, T F, in the range of 0.011 ⩽ T c/T F ⩽ 0.018. These deduced values are very close to the ones reported for H3S at P = 155–165 GPa (Drozdov et al 2015 Nature 525 73). This means that in all considered scenarios the carbonaceous sulfur hydride 190 K superconductor falls into the unconventional superconductor band in the Uemura plot, where all other highly compressed super-hydride/deuterides are located. It should be noted that our analysis shows that all raw R(T,B) data sets for H x (S,C) y samples, for which Snider et al (2020 Nature 586 373) reported T c > 200 K, cannot be characterized as reliable data sources. Thus, independent experimental confirmation/disproof for high-T c values in the carbonaceous sulfur hydride are required.

https://iopscience.iop.org/article/10.1088/1361-6668/abd28e/pdf

The electron-phonon coupling constant, Fermi temperature and unconventional superconductivity in the carbonaceous sulfur hydride 190 K superconductor

Recently Li et al. (2019 Nature 572 624) discovered a new type of oxide superconductor Nd0.8Sr0.2NiO2 with Tc = 14 K. To classify superconductivity in this infinite-layer nickelate experimental upper critical field, Bc2(T), and the self-field critical current densities, Jc(sf,T), reported by Li et al. (2019 Nature 572 624), are analysed in assumption of s-, d-, and p-wave pairing symmetries and single- and multiple-band superconductivity. Based on deduced the ground-state superconducting energy gap, Δ(0), the London penetration depth, λ(0), the relative jump in electronic specific heat at Tc, ΔC/C, and the ratio of 2Δ(0)/kBTc, we conclude that Nd0.8Sr0.2NiO2 is type-II high-κ weak-coupled single-band s-wave superconductor.

Classifying superconductivity in an infinite-layer nickelate Nd0.8Sr0.2NiO2

Fe-based superconductors present a large variety of compounds whose physical properties strongly depend on the crystal structure and chemical composition. Among them, the so-called 1111 compounds show the highest critical temperature Tc in the bulk form. Here we demonstrate the realization of excellent superconducting properties in NdFeAs(O1−xFx). We systematically investigated the correlation between the microstructure at the nanoscale and superconductivity in an epitaxial 22 nm NdFeAs(O1−xFx) thin film on a MgO single crystalline substrate (Tc = 44.7 K). Atomic resolution analysis of the microstructure by transmission electron microscopy and atom probe tomography identified several defects and other inhomogeneities at the nanoscale that can act as extrinsic pinning centers. X-Ray diffraction and transmission electron microscopy displayed a broad variation of the a-axis lattice parameter either due to a partially strained layer at the interface to the substrate, high local strain at dislocation arrays, mosaicity, or due to composition variation within the film. The electrical transport properties are substantially affected by intrinsic pinning and a matching field corresponding to the film thickness and associated with the Bean–Livingston surface barrier of the surfaces. The thin film showed a self-field critical current density Jc(4.2 K) of ∼7.6 MA cm−2 and a record pinning force density of Fp ≈ 1 TN m−3 near 35 T for H‖ab at 4.2 K. These investigations highlight the role of the microstructure in fine-tuning and possibly functionalizing the superconductivity of Fe-based superconductors.

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Microscopic origin of highly enhanced current carrying capabilities of thin NdFeAs(O,F) films

Thin films of Co-doped BaFe$_2$As$_2$ of similar thickness (~40 nm) were grown with different growth rates (0.4 A s$^{-1}$ and 0.9 A s$^{-1}$) by pulsed laser deposition on CaF$_2$(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF$_2$ and a high concentration of planar defects (mainly stacking faults) are observed for the sample grown at a low rate. A higher growth rate results in high-quality epitaxial films with only few antiphase boundaries. A higher $T_\text{c}$ was measured for the sample grown at a low growth rate, which is attributed to the difference in strain state induced by the high concentration of defects. Large crystalline Fe precipitates are observed in both samples. Chemical analysis shows a pronounced O and slight F content at the planar defects which highlights the role of O in defect formation. Electron-beam-induced irradiation damage during TEM measurements is observed and discussed.

https://iopscience.iop.org/article/10.1088/1361-6668/abcecf/pdf

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

The competition between phase formation of BaF2 and Ba(Fe1−x Co x )2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)2As2 thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1−x Te x thin films on CaF2. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-T c cuprate thin films.

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On the growth of Co-doped BaFe 2 As 2 thin films on CaF 2

Quasi-multilayer films with perovskite nano-particles have already been investigated and reported in YBCO compounds. Here, we introduce this technique to iron based superconductors by preparing quasi-multilayer nanocomposite films of Co-doped BaFe2As2 with BaZrO3 and BaHfO3 perovskite nanoparticles by a pulsed laser deposition technique using a frequency-tripled Nd:YAG laser (λ = 355 nm) with separate targets of the constituents. We investigated the enhancement of pinning force density and critical current density in relation to critical temperature reduction. Application-relevant properties are discussed in relation to the deposition conditions and the microstructural properties of the films to better understand growth and behavior of strong pinning centers in these materials.

/pdf/pulsed-laser-deposition-of-quasi-multilayer-superconducting-58rp3jx146.pdf

Sven Meyer

Papers

Microscopic origin of highly enhanced current carrying capabilities of thin NdFeAs(O,F) films

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

On the growth of Co-doped BaFe 2 As 2 thin films on CaF 2

Pulsed Laser Deposition of quasi-multilayer superconducting Ba(Fe0.92Co0.08)2As2-BaHfO3 nanocomposite films

Analysis of superconducting thin films in a modern FIB/SEM dual-beam instrument